<p>The entrySet() method is allowed to return a view of the underlying Map in which a single Entry
 object is reused and returned during the iteration. As of Java 1.6, both IdentityHashMap and EnumMap
 did so. When iterating through such a Map, the Entry value is only valid until you advance to the 
 next iteration. If, for example, you try to pass such an entrySet to an addAll method, things will 
 go badly wrong.</p>